This invention relates in general to voltage controlled oscillators and, in particular, to high-frequency oscillators using a negative resistance.
Numerous types of voltage controlled oscillators appear in the prior art. Typically, the frequency of the oscillation is adjusted by changing the load capacitance of the oscillator circuit. For example, this is the approach used in a standard Colpitts-type oscillator. However, because the capacitances of the various fixed circuit elements combine with the variable capacitance to form the net oscillator capacitance, the net change in th oscillator capacitance is not nearly as large as the change in the variable capacity. Hence, the frequency pullability of such voltage controlled crystal oscillators is limited, especially for practical voltage controlled variable capacitors. The problem can be reduced through choice of large capacitance circuit elements, but at a cost of deteriorated oscillator startability, stability, or crystal drive. Another solution would use the positive reactance of an inductor to null the negative reactance of the circuit capacitances. However, the temperature stability of even the most expensive inductors is generally worse than any other element in the oscillator circuit, so that the temperature stability of the oscillator is degraded. Thus, the inductor solution is both undesireable and expensive to implement. In conclusion, due to the adverse interrelationships between the circuit elements, it is difficult to design a Colpitts-type oscillator for optimum frequency pullability, stability, crystal drive, and startability. Another known approach is to form an oscillator through the use of a negative resistance. The oscillation will occur at a frequency such that the phase shift around a loop consisting of the negative resistance, crystal, and voltage variable capacitance is zero. Usually, the negative resistance is formed by multiplying an appropriate resistor in the circuit by minus one. The prior art discloses this gyration of resistances and general impedances, but these circuits are limited to low-frequency applications in ranges of a few tens of KHz or less.
It is also known that negative resistances can be simulated with the use of operational amplifiers. However, at frequencies upward to 10 MHz, these circuits are no longer practical in performance.
Another problem in the prior art is that in a crystal oscillator, the crystal drive must be set empirically. If the crystal is driven too hard, it dissipates too much energy and the crystal can be damaged. Also, the crystal can deteriorate over a period of time with over driving. Therefore, over driving the crystal can result in increased aging and spurious operation at undesired frequencies.